Wood is comprised of two main components--a fibrous carbohydrate, i.e., cellulosic portion, and a non-fibrous component. The polymeric chains forming the fibrous cellulose portion of the wood are aligned with one another and form strong associated bonds with adjacent chains. The non-fibrous portion of the wood comprises a three-dimensional polymeric material formed primarily of phenylpropane units, known as lignin. Part of the lignin is between the cellulosic fibers, bonding them into a solid mass, although a substantial portion of the lignin is also distributed within the fibers themselves.
For use in paper-making processes, wood must first be reduced to pulp. Pulp may be defined as wood fibers capable of being slurried or suspended and then deposited upon a screen to form a sheet, i.e., of paper. The methods employed to accomplish the pulping step usually involve either physical or chemical treatment of the wood, or a combination of these two treatments, to alter the wood's chemical form and to impart desired properties to the resultant product. There are thus two main types of pulping techniques, i.e., mechanical pulping and chemical pulping. In mechanical pulping, the wood is physically separated into individual fibers. In chemical pulping, the wood chips are digested with chemical solutions to solubilize a portion of the lignin and thus permit its removal. The commonly utilized chemical pulping processes are broadly classified as: (1) the soda process, (2) the sulfite process, and (3) the Kraft process, with the latter process being most commonly used and being capable of a variety of well-known modifications as described below.
The soda process is well known in the art. It employs sodium hydroxide (NaOH) as the active reagent to break down the lignin and to assist in its removal. The sulfite process is also well known in the art (see, e.g., Handbook for Pulp & Paper Technologists--Chapter 6: Sulfite Pulping (TAPPI, U.S.A.).
The Kraft process together with its numerous variations is the principle chemical process utilized in paper manufacturing. The basic Kraft process, as described in the Handbook For Pulp and Paper Technologists--Chapter 7: Kraft Pulping (TAPPI, U.S.A.), involves digesting the wood chips in an aqueous solution of sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2 S). This process is highly effective in the pulping of even difficult woods such as southern softwoods, as well as the other more readily pulped species of wood such as northern hardwoods and softwoods. The Kraft process likewise generally produces a relatively high-strength pulp since its use results in a diminished attack on the cellulose component of the wood.
The modified Kraft techniques can result in even less degradation in the polymeric structure of the cellulosic fibers during pulping and therefore the strength loss in the resultant paper product is diminished as compared to that occurring with the standard Kraft process. One modified Kraft pulping process is known as "extended delignification", which is a broad term used in the art to encompass a variety of modified Kraft techniques, such as adding the pulping chemicals in a specific defined sequence, or at different locations within the digester apparatus, or at different time periods, or with a removal and reinjection of cooling liquors in a prescribed sequence, so as to more effectively remove a greater amount of lignin while reducing the severity of the pulping liquor's chemical attack on the cellulosic fibers. Another modification of the Kraft process is the Kraft-AQ process, wherein a small amount of anthraquinone is added to the Kraft pulping liquor to accelerate delignification while limiting the attack upon the cellulosic fibers which comprise the wood.
A variety of additional extended delignification techniques are known in the art and include Kamyr Modified Continuous Cooking (MCC) as described by V.A. Kortelainen and E. A. Backlund in TAPPI, vol. 68 (11), 70 (1985); Beloit Rapid Displacement Heating (RDH) as reported by R. S. Grant in TAPPI, vol. 66 (3), 120 (1983); and Sunds Cold Blow Cooking as reported by B. Pettersson and B. Ernerfeldt in Pulp and Paper, vol. 59 (11) 90 (1985).
Digestion of the wood by a Kraft or modified Kraft process results in the formation of a dark colored slurry of cellulose fibers known as "brownstock". The dark color of the brownstock is attributable to the fact that not all of the lignin has been removed during digestion and has been chemically modified in pulping to form chromophoric groups. Thus, in order to lighten the color of the brownstock pulp, i.e., to make it suitable for use as printing and writing and other white paper applications, it is necessary to continue the removal of the remaining lignin by the addition of delignifying materials and by chemically converting any residual lignin into colorless compounds by a process known as "bleaching" or "brightening".
Prior to bleaching the pulp, however, the digested material is conventionally transferred to a separate blow tank after the chemical treatments involved in the pulping process are completed. Within the blow tank, the pressure developed during the initial chemical treatment of the lignocellulosic material is relieved and the pulp material is separated into a fibrous mass. The resulting fibrous mass is then subjected to a series of washing steps to remove the combination of any residual chemicals and the soluble materials (such as the lignin) which were separated from the fibrous materials in the pulping process. Frequently, the pulp also undergoes one or more screening steps designed to separate out the larger portions of undefibered wood for special processing (recooking, mechanical grinding, etc.).
The residue obtained from the washing process, commonly referred to as black liquor, is collected, concentrated, and then incinerated in an environmentally safe manner in a recovery boiler. The technique for the collection, concentration and burning of the black liquor is conventional and is well known in the art.
The delignification and bleaching processes are conducted on the washed fibrous mass in a series of steps, using selected combinations of chemical reactants. In the prior art, various combinations of chemical treatments have been suggested. Furthermore, individual treatment steps have been rearranged in an almost limitless number of combinations and permutations. Therefore, in order to simplify the explanation of the various bleaching processes and systems, the use of letter codes is conventionally employed in combination to describe the particular chemical reactants employed and the sequence of the steps of the process.
The letter codes which will be used hereafter, where appropriate, are as follows:
C=Chlorination--Reaction with elemental chlorine in acidic medium. PA1 E=Alkaline Extraction--Dissolution of reaction products with NaOH. PA1 Oxidative Alkaline Extraction--Dissolution of reaction products with NaOH and Oxygen. PA1 D=Chlorine Dioxide--Reaction with ClO.sub.2 in acidic medium. PA1 P=Peroxide--Reaction with peroxides in alkaline medium. PA1 O=Oxygen--Reaction with elemental oxygen in alkaline medium. PA1 O.sub.m =Modified Oxygen--Uniform alkali treatment of low to medium consistency pulp followed by reaction of high consistency pulp with oxygen. PA1 Z=Ozone--Reaction with ozone. PA1 Z.sub.m =Modified Ozone--Uniform reaction with ozone. PA1 C/D--Admixtures of chlorine and chlorine dioxide. PA1 H=Hypochlorite--Reaction with hypochlorite in an alkaline solution. PA1 chemically digesting a lignocellulosic material to initially form a pulp; PA1 oxygen delignifying the pulp to remove a substantial portion of the lignin therefrom, with the combination of the digesting and oxygen delignifying steps being conducted to form an intermediate pulp having a specified amount of lignin and a specified viscosity; and PA1 ozone delignifying the intermediate pulp with a gaseous mixture that contains ozone by adjusting the consistency of the pulp to a high consistency of above about 20%, adjusting the pH of the pulp to below about 4, and treating the pulp with an amount of the ozone containing gaseous mixture sufficient to remove a substantial portion, but not all, of the remaining lignin by intimately contacting and turbulently mixing the pulp particles with the gaseous mixture in a dynamic reaction zone for a sufficient time and at a temperature sufficient to allow access of the ozone to substantially all of the pulp for reaction therewith while the pulp advances through substantially all of the reaction zone, thus obtaining substantially uniform delignification of a significant portion of the pulp and forming a delignified pulp having a reduced amount of lignin and the certain strength, viscosity and GE brightness. PA1 introducing the high consistency pulp into the reaction zone at a fill level of at least about 10%; PA1 introducing the ozone containing gaseous mixture into the reaction zone for contact with the pulp; and PA1 intimately contacting and mixing the pulp with the ozone by lifting, displacing and tossing the pulp in a radial direction to disperse the pulp and expose substantially all of the pulp to the gaseous bleaching agent while advancing the dispersed pulp axially through the reactor in a plug-flow like manner and at the desired dispersion index for a predetermined time to obtain substantially uniform bleaching of the pulp and to form a bleached pulp having the certain GE brightness, certain strength and certain viscosity. PA1 increasing the consistency of the intermediate pulp to at least about 28%; PA1 comminuting the increased consistency pulp into discrete particles of a predetermined particle size having a sufficiently small diameter and a sufficiently low density to facilitate substantially complete penetration of a majority of the pulp particles by ozone gas without causing significant degradation of the cellulose components of the pulp; and PA1 uniformly contacting the comminuted pulp particles and the ozone containing gaseous mixture during the turbulent mixing step while the pulp is advanced through the reaction zone for a sufficient time to obtain substantially uniform delignification of a majority of the pulp particles. PA1 optionally applying a second amount of alkaline material onto the high consistency pulp to obtain a total amount of alkaline material on the pulp of at least about 0.8 to 7 percent by weight based on the oven dry weight of the pulp, wherein at least about 55% to 100% of the total amount of alkaline material is applied to the pulp during the alkaline material combining step; and PA1 subjecting the increased consistency alkaline material containing pulp to high consistency oxygen delignification to obtain enhanced delignification of the pulp without a corresponding decrease in pulp viscosity compared to pulp which is not combined with alkaline material at low consistencies and form an intermediate pulp having a specified amount of lignin and having a specified viscosity. PA1 contacting, in an initial bleaching stage, the delignified pulp with a sufficient amount of at least about 0.3% weight of a peroxide compound for a sufficient time of between about 2-15 minutes with substantially constant mixing to raise the intermediate brightness of the delignified pulp by at least about 7 GEB points; and PA1 contacting, in a final bleaching stage, the increased brightness pulp with a sufficient additional amount of at least about 0.4% by weight of a peroxide compound for up to about three hours to further raise the GE brightness of the pulp to at least about 83. If desired, the effluent from the final bleaching stage may be recycled to the initial bleaching stage to reduce the total fresh peroxide requirement.
O.sub.m and Z.sub.m are modified processes according to the present invention and are described further in the